Resize the red builder brush so that it covers everything that you wish to flood. It is important to match the top face of the red builder brush exactly with the plane you've just placed, or else the water would actually end at a location other than what the plane would imply. The plane's only use is to visually represent a body of water. The real water, however, is entirely independent of the plane. It would be possible to delete the plane and still have water to swim in, albeit invisible water.
Once you have positioned the red builder brush correctly, right-click the volume button on the left toolbar and pick UTWaterVolume.
Now, move the red builder brush away to reveal the pink water volume below. If you do not see ...
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